Three prognostic factors have been associated with worse outcome in sickle cell disease:
1.) Dactylitis in infants younger than age 1 year
2.) Hemoglobin level less than 7 g/L
3.) Leukocytosis in the absence of infection
These worse outcomes include death, risk of stroke, high pain rate, recurrent acute chest syndrome.
Check out the article here: http://www.nejm.org/doi/full/10.1056/NEJM200001133420203